M-values clarification please

[applepie]

Right, bear with me. I am slow on the update. so wanted to clarify a few things

I am not 100% about what they represent in real terms, I am pretty sure when ascending you should not be above the line else potential bend/micro-bubble formation. To be most efficient off-gas, you should follow quite close to the line (I was taught this but disagree-ish somewhat)

I understand the principle behind it based on Haldanes 2:1 pressure ratio guide.

Now on the graph, you have tissue tension on the Y axis and stop depth on the x axis. I have taken tissue tension to mean at what depth your tissues are? (hence subjected to the pressure/tension at that depth)

so to determine your ceiling, you read off you depth trace it to the line that represents your tissue half time and voila you have your max ceiling. correct?

I am quite sure that the above is correct from what I recall. just wanted to check.

how did they calculate the lines originally then? was it a theoretical mathematical calculation or based on experiments on squaddies or similar.

So by slowly recalculating your depth and determining your M-value, you generate your decompression profile. I think most computers use this principle?

But this doesnt take into account the time factor right? How do you add that in?

I would guess that the fast tissue is the limiting factor to begin with, however, does the slow tissue start to be the major factor at some point? Hard to explain what I mean. Initially your ceiling will be determined by the fast tissue (m-values) but at some point does the slow tissue determine the ceiling? Since the fast tissue is able to off-gas faster, and the slow tissue will have residual. or does it only affect repetitive dives? (which I think so from what I understand).

hmm finding some of this decompression stuff a bit complicated cant wrap my head around some principles properly. its annoying since its like being halfway there but your brain just doesnt come up with a clear solution.

[Mal Bridgeman]

Hi ... not sure what training you have already had, but this, from YD's own NigelH is a good read and explains what you want to know, I think
Mal

[chrisch]

Originally Posted by applepie

...
I am not 100% about what they represent in real terms, ...

M values are the theoretical maximum over pressure of inert gas. A function of pressure and time. (one half time - half saturated, six half times fully saturated).

The "line" you refer to is Robert Workman's idea and allows the original Haldane calculations to be calculated easily. There is one line per tissue group.

Tissue tension is the level of gas pressure not the depth. I think this is where you are getting confused. Try to forget the wierd concept of "depth"

I'll try to give you an example but we are in danger of confusing one another

Take one tissue group and saturate it for one half-time at a given ambient pressure of - for example - 3 ATA (c20msw). With air that is 3ATA*0.79*0.5

1.185ATA - not a depth.

Chris

[Freeflow]

M values are given in MSW [metres of sea water] to simplify the pressure calculations